when found.configfn return false, m is null

example, I want to use patterns like this:
```
patterns : {
        "xxx-": {
               configFn: function(mod) {
                     //I don't want to load this mod, but don't want to use test
                     return false;
               }
       }
}
```
then, execute this code: 
```
ret = o.configFn(o);
            if (ret === false) {
                Y.log('Config function returned false for ' + name + ', skipping.', 'info', 'loader');
                delete this.moduleInfo[name];
                delete GLOBAL_ENV._renderedMods[name];
                o = null;
            }
```
yui.js throw error,  if code allow configFn return false，the after code should be compatible。